Ancient loss of gene function across ancestral marine mammal lineages may now be putting modern marine mammals at risk, leaving them defenseless against toxic organophosphates. According to new research, the shared, or convergent, genetic loss of Paraoxonase 1 (PON1) has left many marine mammal species without a mechanism to break down these neurotoxic chemicals, which are increasingly finding their way into their marine habitats. When land mammals returned to the sea, millions of years of adaptive changes allowed them to colonize the planet's oceans. Some physiological and morphological changes occurred similarly across multiple distinct lineages of marine mammals, like the development of flippers in whales, seals and sea cows. As part of such processes, associated changes at the gene level, however, can influence more than one potentially unrelated trait, leading to unforeseen outcomes in changing environments. Wynn Meyer et al. conducted a genome-wide scan for shared losses of gene function across marine mammal species, related to their ancestral transition to aquatic environments. The authors revealed a striking pattern of convergent loss at the gene PON1, mammals' lone defense against the highly toxic organophosphate compounds found in the heavily used pesticide chlorpyrifos. The gene lost function independently in all three marine mammal lineages (cetacean, pinniped and sirenian) the authors evaluated, but remained intact in terrestrial mammal genomes. According to Meyer et al., the loss of PON1 may be related to its role in fatty acid oxidation. The results underscore the potential health risks for marine mammals that live close to agricultural runoff containing organophosphorus pesticides, like manatees and dugongs.